1. Field of the Invention
The present invention relates to lapping, polishing, finishing or smoothing of surfaces with apparatus and processes which use abrasive sheeting. In particular, the present invention relates to such processes and apparatus, preferably those which use removable or replaceable abrasive sheeting, which operates at high surface speeds, and preferably which secures the abrasive sheeting to a platen on a flexible shaft, which platen moves the sheeting at those high speeds. The lapping system is capable of extremely smooth surface finishing at high speeds.
2. Background of the Art
The field of lapping or polishing traces it roots far back into time, even before substantial technical developments. Early jewelry and decorations were provided by minerals or materials (shells or wood) that had been smoothed by natural elements. Stones smoothed by water currents or sand storms gave a much more pleasant look and feel than unpolished stones or stones which had been roughly smoothed by available means such as rubbing two stones together.
Early efforts at sharpening blades for plows or swords were amongst the first technical advances in lapping and smoothing of materials, and these technical means are still used in much the same way today. Swords and plow shears were sharpened by moving the blade against a stone surface. The abrasive action of the stone against the blade removed metal and thinned the blade at its edge. Grinding wheels, kitchen knife sharpeners, and the like are not significantly different in function than the stone sharpening tools, such as the grinding wheel which has been used to sharpen blades for thousands of years.
In the 17.sup.th and 18.sup.th century, the combination of die casting and abrasive polishing enabled the manufacture of interchangeable generic parts for equipment (especially the rifle and hand gun) as opposed to the standard method of fitting individually made parts into a unique piece of equipment with uniquely fitting parts. Each succeeding advance in the ability of materials and processes to create smoother and more uniform surfaces advanced the quality and capability of the resultant articles to perform whatever tasks for which they were designed. Lenses with greater smoothness and uniformity advanced the degree to which observation could be extended downward by microscopy and outward into space by telescopes. Better fitting parts extended the longevity of equipment and increased efficiency by reducing internal friction. The need for increasing efficiency, precision, consistency and smoothness in lapping is as important today as ever. Each incremental increase in the quality of lapping materials and processes advances many fields of technology and industry, while at the same time offering the possibility of reducing the cost of manufacture of goods.
Lapping and polishing are performed in many fields and industries. Metal and parts polishing is the most obvious field, but smoothing of surfaces is extensively used in lens manufacture, semiconductive wafer manufacture, gem polishing, preparation of supports for optical elements, providing surfaces which can be joined or seamed and the like. The smoothness and reproducibility of the processes and apparatus used to create the needed levels of smoothness are critical to the success of products. U.S. Pat. No. 5,584,746 (Tanaka) describes a method of polishing semiconductor wafers and apparatus therefor. The import of Tanaka is the physical control placed over the wafer as it is being polished. The wafer is secured by a vacuum system on a wafer mounting plate. The relative flexibility of the wafer is discussed as a method of controlling uniformity of the wafer surface as is the uniformity of the vacuum applied through the wafer support. The polishing of the wafer surface is accomplished by typical means including a polishing pad which is mounted on a polishing surface (turntable). It is suggested that the pad should not be subject to plastic deformation and may be preferably selected from a group comprising close cell foam (e.g., polyurethane), polyurethane impregnated polyester non-woven fabric and the like, which are known materials in the art. No specific means of securing the polishing pad to the support surface is described in Tanaka. No specific speeds of rotation for the operation of the process are shown in the examples.
U.S. Pat. No. 5,317,836 (Hasegawa) describes an apparatus for polishing chamfers of a wafer. Hasegawa describes that in the manufacture of wafer materials from single crystal ingots such as silicon, the wafer is produced by a combination or selection of processes including slicing, chamfering, lapping, etching, buffing, annealing and polishing. It is noted that chipping and/or incomplete surface polishing are a problem in such ingot conversions to wafers. Hasegawa describes the use of a rotary cylindrical buff formed with at least one annular groove in its side describing a circle normal to the axis of the cylindrical buff and a wafer holder capable of holding and turning the wafer about an axis. The improvement is described as including at least the ability of the cylindrical buff being adapted to freely shift axially, that the annular groove has a width substantially greater than the thickness of the wafer, and that the apparatus further comprises a means for axially biasing the cylindrical buff. No specific speeds of rotation for the operation of the process are shown in the examples.
U.S. Pat. No. 5,007,209 (Saito) describes an optical fiber connector polishing apparatus and method. Saito describes a method and apparatus for polishing optical fiber connectors with high accuracy. Saito indicates that the polishing is accomplished by using an elastic polishing board rotating at high speed, but no specific speed of rotation or method of attachment of the polishing board is described. Positioning pins and other controls are provided in the system to accurately align the swing fulcrum arm carrying the polishing material.
U.S. Pat. No. 4,085,549 (Hodges) describes a lens polishing machine comprising a lap tool holder and lens blank holder including independent means to provide linear and rotary movement between a lens blank and lap tool. The machine is described as useful for high speed grinding and polishing. The polishing element is gimbal mounted on its lower extreme in a spherical bearing to allow a lens blank holder to follow the contour of the lens during the polishing process. The movement between the rotary drive and linear drive mechanisms independent of each other provides a balanced and low vibration operation. No specific speeds of rotation are recited and the abrasion is provided by a slurry.
U.S. Pat. No. 4,612,733 (Lee) describes a very high speed lap with a positive lift effect. The apparatus and method comprises a rotary lapping system which uses a liquid slurry of abrasive particles. The diameters of the particles are shown to be from about 1.5 to 5 micrometers, but may be outside this range. The system is described as producing positive lift by presenting leading edge surfaces with a positive angle of attack in the liquid abrasive slurry, the leading edge surfaces generating a positive lift through hydrodynamic interaction with the slurry. Each of the positive lift tools presents a grinding surface to said workpiece when it is rotated in the slurry. There is again no specific rotational speed provided in the description, and the use of liquid slurries would cause higher lapping/abrasive areas on the exterior of the grinding/lapping face as the slurry would be at higher levels at the outside of the rotating grinding area work surface.
U.S. Pat. No. 4,709,508 (Junker) describes a method and apparatus for high speed profile grinding of rotatably clamped, rotation symmetrical workpieces. Rather than the grinding element contacting the surface to be ground with a grinding surface which is rotating within a plane, the edge of the grinding element (e.g., at the circumference of a disk rather than on its face) is brought against the surface to be ground.
U.S. Pat. No. 5,197,228 describes methods and apparatus for grinding metal parts, especially with devices having a cooperative workpiece holder and a tool holder which form a grinding station. The grinder table is reciprocally moveable along an axis which is at right angle to the axis of travel of the workpiece. The grinder table may also be equipped for controlled simultaneous movement along two axes. A microprocessor is designed to send and receive signals to or from all of the moving parts of the grinding machine for moving the workpiece table towards or away from the grinding bit.
U.S. Pat. No. 4,194,324 describes a carrier for semiconductive wafers during polishing steps in their manufacture. An annular flange is present to receive pressure loading from the polishing machine during the wafer polishing operation. The holder of the polishing machine includes the ability to apply a vacuum to the carrier to maintain the carrier selectively on the polishing machine. The arrangement on the equipment allows release of the vacuum during polishing and enables simple intentional removal of the carrier. Cam follower-slot arrangements permit tilting of the mounting head.
U.S. Pat. No. 5,576,754 describes a sheet holding device for an arcuate surface with vacuum retention. The sheet and device are described as useful for internal drum plotters in imaging equipment. Vacuum pressure is applied to imaging film to keep it securely positioned within the arcuate focal plane of the imaging equipment.
U.S. Pat. No. 5,563,683 describes a substrate holder for vacuum mounting a substrate. The holder is provided with two kinds of grooves or clearances in the supporting surface. Circular support faces with multiple grooves and/or a plurality of pins to support the work are shown. The device is generally described to be useful as a holder, with such particular uses as in the manufacture of semiconductors and the support of photosensitive substrate being shown. Similarly, U.S. Pat. No. 4,943,148 describes a silicon wafer holder with at least one access port providing access to the underside of the wafer with vacuum pressure. U.S. Pat. No. 4,707,012 also describes a method of applying vacuum holding forces to a semiconductor wafer during manufacture in an improved manner. U.S. Pat. No. 4,620,738 shows the use of a vacuum pickup system for semiconductor wafers. The wafers are placed into or removed from holders by the vacuum pickup.
Similarly, U.S. Pat. No. 5,414,491 describes a vacuum holder for sheet materials comprising a plurality of arrays of vacuum channels including a plurality of vacuum plenums. Flow sensors are provided so that the system can indicate the presence and/or size of the sheets being held. Specifically described are common types of imaging materials using sheets of plain paper, photographic paper and photographic film.
U.S. Pat. No. 5,374,021 describes a vacuum holding system which is particularly useful as a vacuum table for holding articles. The holding table is particularly described with respect to the manufacture of printed circuit boards. Controlled passageways are provided which are supposed to control the application of reduced pressure and to reduce the application of the vacuum when vacuum support is not required.
U.S. Pat. No. 5,324,012 describes a holding apparatus for holding an article such as a semiconductor wafer. At least a portion of the holder contacting the wafer comprises a sintered ceramic containing certain conductive materials. The use of conductive materials and fewer pores reduces the occurrence and deposition of fine particles during use. The benefits of the materials are said to be in contributions to the cleanability of the surface, insurance of mechanical strength, reduction of weight and increased dimensional stability.
U.S. Pat. No. 5,029,555 describes a holding apparatus and method for supporting wafers during a vacuum deposition process. The apparatus is an improved system for the angled exposure of at least one surface portion of a substrate supported on a surface holder to an emission of a source impinging obliquely on the surface portion. The device moves the surface holder to improve the uniformity of the emission received on the surface portion. Wheel mechanisms are coupled together to provide maintenance capability for predetermined positions of the surface. The substrate holder is moved while its orientation to the source is carefully controlled.
U.S. Pat. Nos. 4,483,703 and 4,511,387 describe vacuum holders used to shape glass. Frames are shown with slidable members moving a deformable vacuum holder between a shaping station and a mold retraction station. Pistons drive movable elements, such as the vacuum holder, on a supporting frame.
U.S. Pat. No. 4,851,749 describes a motor driven mechanical positioner capable of moving an arm to any one of about 840 discrete angular positions. An infrared light emitting device acts with a phototransistor to control the appropriate angular position. Sensing devices also act on interdependent speed controls so as to increase the accuracy of the positioning of the arm.
U.S. Pat. No. 5,180,955 describes a positioning apparatus comprising an electromechanical system which provides controlled X-Y motion with high acceleration, high maximum speeds, and high accuracy, particularly for positioning an end-effector at predetermined locations. A high speed mini-positioner is provided comprising a positioning linkage having a changeable parallelogram structure and a base structure. A main benefit of the system is the fact that the bars and bearings of the positioner are symmetrical about the X-Y plane passing through the linkage height. The symmetry means that all actuator forces and all inertial reaction forces act in vectors lying in the plane of symmetry.
U.S. Pat. No. 5,547,330 describes an ergonomic three axis positioner. The positioner is intended to move an article along three mutually perpendicular axes through a system of interconnected slides and slide joints. Rack and pinions are also used to independently move the slides. The device is suggested for use in the visual inspection of work, particularly in the semiconductor industry.
U.S. Pat. No. 4,219,972 describes a control apparatus for a grinding machine. A revolution speeds changing means is provided which can selectively effect changes at high speeds when grinding and changes at low speeds when dressing the article. The relationship and control of the timing of the speed changes and the operations detection circuits and timers.
U.S. Pat. No. Re. 30,601 describes an apparatus and method particularly effective in the positioning of a semiconductor wafer in a preferred plane with respect to a photomask. Sensors regularly monitor the position of the wafer and a reference plane. A photoalignment system is provided in which a wafer is not physically touched by any portion of the photoalignment tool, thereby avoiding any contamination.
These systems have been described as providing benefits to particular technical and commercial fields, but they have not been shown to provide any particular benefits to truly high speed lapping/polishing systems and materials.
U.S. patent application Ser. No. 09/036126, filed on Mar. 6, 1998 describes advances in the performance of lapping technology, including at least the following technology. Lapping or polishing at high speeds with fine abrasive particles offer significant advantages in the speed of lapping, savings of time in lapping, and smoothness in the finished articles. Materials, processes, apparatus and specific features integrated into the lapping processes and apparatus of the present invention can provide a unique lapping effect with regard to both the quality (smoothness and uniformity of the produced surface) and efficiency of the system. The invention of U.S. patent application Ser. No. 09/036126 relates to a new field of lapping technology with its own unique complexities due to the combination of high rotational speeds on the abrasive platen and the use of sheets of abrasive material rather than slurries. The combination of these two aspects creates dynamics and forces which have not been addressed by previous lapping systems and requires an entirely new background of engineering to address the problems.
Another aspect is a preferred process within the scope of the invention which comprises:
a) providing a work piece to be lapped, said work piece having a first surface and a second surface which are parallel to each other, and at least one of said first and second surface is a surface to be lapped, PA1 b) providing a first and second rotating platen, each of said first and rotating platen having I) a back surface and ii) a flat front surface which can be adjusted so that said first platen is facing and parallel to said first surface of said work piece and said second platen is facing and parallel to said second surface of said work piece, PA1 c) providing a sheet of abrasive material on said flat surface of said first platen with an abrasive face of said sheet facing said first surface of said work piece which is said at least one surface to be lapped, PA1 d) securing said sheet of abrasive material to said flat surface of said first platen, and PA1 e) putting a liquid between both I) said first platen and said first surface of said work piece and ii) said second platen and said second surface of said work piece, PA1 f) rotating both of said platen at at least 500 revolutions per minute and contacting said abrasive material and said work piece, PA1 g) wherein contact pressure between said both I) said first platen and said first surface of said work piece and ii) said second platen and said second surface of said work piece are sufficiently similar that said work piece does not flex more than 0.1 mm at its exterior regions between said two platens. PA1 a) supporting a workpiece on a workpiece holder, PA1 b) supporting said workpiece holder on a linearly movable support, PA1 c) advancing the workpiece into contact with an abrasive surface comprising abrasive sheeting on a rotatable platen, PA1 d) determining a position at least approximating the position of contact between a surface of said workpiece to be ground and said abrasive surface, PA1 e) removing said workpiece from said position approximating the position of contact, PA1 f) advancing the workpiece towards said abrasive surface while said rotatable platen is rotating, and PA1 g) controlling forces which advance said workpiece towards said abrasive surface and into contact with said abrasive surface. PA1 a) providing a work piece to be lapped, having at least one surface to be lapped, PA1 b) providing a rotatable platen having a back side and a front side, said front side facing said work piece and having a flat plateau which is continuous around the perimeter of said front side of said rotatable platen and is elevated with respect to a central area on said front side, PA1 c) providing a sheet of abrasive material on said flat plateau, said sheet of abrasive material having a front surface with an abrasive face and a back surface, with said abrasive face facing said at least one surface to be lapped, PA1 d) securing said sheet of abrasive material to said flat surface of said plateau, and PA1 e) rotating said platen at at least 500 revolutions per minute and contacting said abrasive material and said work piece to remove material from said work piece. PA1 a) a shaft which is connected to a rotatable platen having vents for air on a front surface of said platen, said platen having a back side to which said shaft is connected and a flat front side on said platen to which can be secured an abrasive sheet by reduced air pressure conveyed through said vents; PA1 b) a frame having a total weight of at least 200 kg supporting a work piece holder and said shaft connected to a rotatable platen; PA1 c) a work piece holder which is movable on said frame; PA1 d) said work piece holder being attached to a movable element on said frame which is capable of moving along said frame in a direction towards and away from said platen to perform lapping of a work piece held on said work piece holder; PA1 e) said work piece holder having control element thereon which allow for independent movement and alignment of said work piece holder along three perpendicular axes so that a work piece on said work piece holder can be adjusted and oriented towards parallel orientation (variously referred to herein as parallelity or parallelism) parallelity with said platen so that a work piece can be lapped; and PA1 f) most preferably said control elements having at least 50 settings per rotation, each setting moving said shaft along one of said three axes by a dimension less than 0.05 mm. PA1 a) a frame having a total weight of at least 200 kg supporting a work piece holder PA1 b) a rotatable platen having an abrasive surface comprising an abrasive sheet secured to said platen; PA1 c) a work piece holder which is movable on said frame; PA1 d) said frame being movable in three dimensions, with controls for each of the dimensions of movement (e.g., hinges, positioning screws, hydraulics, electric motors, etc), PA1 e) walls may be present above a plane defined by a surface on said rotatable platen which carries abrasive; and PA1 f) said rotatable platen being surrounded on all sides by said walls which may be angled (over said plane and towards said platen) to deflect impacting material downward or at least preventing impacting material from ricocheting upwardly out of the impact area (e.g., by using extensions or lips from the walls which overlay the impact area and prevent vertical ricocheting off of the walls). PA1 g) a means for introducing a second amount of liquid onto said abrasive surface of said platen after contact between said work piece and said abrasive surface; and PA1 h) means for directing air against said abrasive surface after introduction of said second amount of liquid.
A very important process aspect of that invention includes the initial positioning and contacting of the workpiece and the abrasive sheet material as in a process for initiating contact between a workpiece to be ground and an abrasive surface comprising abrasive sheeting on a rotatable platen, the process comprising:
said process being further characterized by
In that process, mechanical alignment of said workpiece and/or said workpiece holder is effected to promote parallelism between a surface of said workpiece to be ground and said abrasive surface after step c) but before step e). The controlling forces provides a preferred contact force between 0.1 and 10 pounds per square inch between a surface of said workpiece to be ground and said rotating platen during lapping of said workpiece while said abrasive sheet is moving with at least 1,500 surface feet per minute while in contact with said workpiece. This process and lapping system has the workpiece holder supported by a pivot joint and said workpiece holder pivots upon contact between said workpiece and said abrasive surface to hold a surface of said workpiece to be lapped in a more parallel orientation with said abrasive surface. Another desirable aspect of the process of the present invention is that pressure is applied between the work piece and the abrasive sheet by a flexible joint or engagement or gimbal supporting the work piece. The pressure applied between the workpiece and the rotating platen may be from 0.1 psi to 100 psi, preferably from 0.1 to 25 psi, more preferably from 0.1 or 0.5 to 5 psi.
Generally a particular improved process of that invention may be considered to comprise a process for lapping a surface comprising:
That process particularly benefits when the plateau defines an annular shape on said front face, and more particularly where the sheet of abrasive material comprises a circular sheet or annular sheet of material. The sheet of abrasive material most preferably comprises an annular shape in which a central open portion is at least three times the radial dimension as the width of said annular sheet. A reduced gas pressure may be applied against said back surface of said sheet between said sheet and said platen through vents which are present at least or only on said flat surface of said plateau, the reduced pressure securing the sheet against rotational movement relative to the rotatable platen. A preferred abrasive sheet comprises an annular distribution of abrasive material on a backing material, with a center area of said sheet being a self-supporting structure which passes across said center area, contacting inner edges of said annular distribution of abrasive material. That is, the central area may be free of abrasive material, such as where said abrasive sheet comprises a continuous substrate with a central area having no abrasive on said backing material, and an annular zone of said backing material surrounding said central area having abrasive material on a surface overlaying said plateau and facing away from said platen, or where said abrasive sheet comprises an annular zone and said central area, said central area being bonded to said annular zone, having less height than said annular zone when said sheet is lying flat, and there being a seam or bond between said annular zone and said central area.
A preferred lapper platen system according to that present invention may comprise:
Movement and control of movement of the workpiece holder can be extremely important in the performance of the present invention. The control of the movement is best effected by the use of support systems for the workpiece which allow smooth motion of the workpiece, especially by air pressure, hydraulic pressure, linear electric motors and the like.
Another improved process for lapping a surface according to that invention comprises:
using a lapper system comprising:
It is preferred that a safety box system is also included within the lapping system which may include a means for introducing a first amount of liquid onto said abrasive surface of said platen at a location before contact between a work piece held on said work piece holder and said abrasive surface on said platen;
The second amount of water is larger than the first amount, the first amount providing a function as a lubricant, coolant, or the like, and the second amount assisting in washing away residue from the work piece and/or the abrasive sheet. The means for directing air against the abrasive surface of the platen assisting in the rapid removal of the liquid and the solid matter carried with it.
There are extreme and diverse forces present within that lapping system, with high speed, surface to surface contact, highly abrasive contact, liquid flow, torque forces, centrifugal forces and the like. Many of these forces operate against the primary objectives and needs of the lapping system, in keeping the rotating platen and work piece surface in level and uniform contact with each other. Controls and modifications of the original equipment have to be made to assure improvement of the performance of the original equipment.